This is probably due to different vendors and operators using totally different retry schemes. Keep in mind different vendors/platforms have different features and parameters implemented to handle their queues.
What exactly do you want to know?
I would start by reading the very few lines ETSI put up regarding SMS Retry (4.4 in TR 102 529 - V1.1.1) and relate that to the relevant specifications for SMS Delivery. Also, some vendors have put out general information about their parameters, eg. Symsoft and opensource Kannel/EasySMS.
Also find a patented retry algo from http://www.google.com/patents/US8055287.